Estrogen sulfotransferase (EST) is a 35 kd cytosolic enzyme that catalyzes specifically the sulfo-conjugation and inactivation of estrogen. A primary site for Est expression in mammals is the Leydig cells of the testis. The central objective of this proposal is to test the hypothesis that EST plays an important role in normal testicular function by modulating the activity of locally synthesized estrogen in the testis. The specific aims of this proposal are: 1. To test the protective role of EST in estrogen-induced steroidogenic lesions in cultured leydig cells by Est cDNA transfection studies 2. To generate Est knock-out mice through homologous recombination in embryonic stem cells and to examine if key testicular functions including androgen biosynthesis and spermatogenesis are impaired by unattenuated estrogen activity 3. To determine if the expression of Est in Leydig cells is regulated by LH in the same fashion as testicular steroidogenesis and whether the effect of LH is direct or involves the action of androgens.